1. Field of the Invention
The present invention relates to electronic signal metrology technology. In particular, the present invention relates to logarithmic detectors or amplifiers.
2. Description of the Prior Art
Logarithmic detectors or amplifiers are used to measure signals having a large dynamic range. For example, applications requiring compression of a wide range of analog input data and linearization of transducers having exponential outputs. Logarithmic amplifiers are used mainly in communication applications for measuring receive signal strength indicator (RSSI) and for controlling the radio frequency (RF) power transmitted in a power amplifier. A logarithmic amplifier (logamp) is a device that represents RF signals at its input by an equivalent decibel-scaled DC voltage at its output. FIG. 1A shows the output of a typical logarithmic amplifier at 10. An ideal response of a logamp is shown as a straight line 12 when plotted against a logarithmic/decibel scaled x-axis. This ideal response is approximated by the successive compression of a cascade of amplifiers. For small input signals a cascade of amplifiers will have large combined gain that progressively diminishes as larger input signals force latter stages of the cascade of amplifiers into compression. Increasing the gain increases the sensitivity of the logamp to small input signals. The actual response of a three stage logamp is shown as a series of three curves 14 when plotted on the same graph.
FIG. 1B shows the outputs of FIG. 1A plotted on a linear graph at 20. The ideal response of the logamp is shown as a curve 12 when plotted on a linear scale. An actual response of a three stage logamp is shown as line segments 14. As can be seen from FIG. 1B the deviation of the actual response 14 from the ideal response 12 can be reduced by simultaneously increasing the number of stages in the logamp and reducing the gain of each stage such that the small signal gain remains constant. Such action would result in a greater number of segments of the actual response curve 14 (FIG. 1B) and thereby reduce deviation between the two curves 12 and 14.
FIG. 2 shows the most common circuit implementation of a logamp at 50. This configuration is referred to as a current mode approach. A voltage in 52 is applied to a cascade of amplifiers 54. The voltage at the output of each amplifier 56 is converted to current at V/I converter 58. This current is rectified by rectifier 60. The rectified currents from all of the amplifiers 56 is summed across resistor 62, which after filtering results in a decibel-scaled DC voltage at the output node 64. Small input signals 52 will produce small combined rectified currents because only latter stages of the cascade of amplifiers 54 will convert the voltage into current. These small rectified currents will result in a small DC output voltage at output node 64. A large input signal will cause a larger number of currents to sum onto the output node 64 thereby producing a large DC output voltage.
The implementation shown in FIG. 2 is relatively insensitive to temperature variations. Each of the gain stages in the cascade 54 is biased with a proportional to absolute temperature (PTAT) current source and the combination of V/I converter 58 with the rectifier 60 is biased with a constant current source derived from a bandgap reference. In this way, the voltage at the output of each amplifier 56 remains constant despite changes in temperature, which results in constant current at the output of each rectifier 60 and an overall output voltage that is insensitive to temperature.
One problem with the current mode amplifier described above is that it can only function in a relatively limited bandwidth due to the use of current rectifiers 60. Another problem with the current mode amplifier is that such a device consumes a relatively large amount of current to operate.
Therefore, it is desirable to provide logarithmic amplifier that operates at a broad range of input frequencies. Furthermore, it is desirable to provide a logarithmic amplifier that consumes less current than current mode logarithmic amplifiers.
The present invention teaches a logarithmic amplifier that operates at a broad range of input frequencies. The present invention also teaches a logarithmic amplifier that consumes less current than current mode logarithmic amplifiers.
A first embodiment of the present invention teaches a voltage mode logarithmic amplifier comprising: at least one first gain stage for providing at least one amplified rectified voltage signal at least partially responsive to at least one input voltage signal; at least one second gain stage for providing at least one further amplified rectified signal at least partially responsive to the at least one input voltage signal; and at least one output node for producing at least one output voltage signal that is at least partially responsive to the at least one amplified rectified voltage signal and the at least one further amplified rectified voltage signal.
The voltage mode logarithmic amplifier further including: at least one self-biased replica stage operative to provide at least one voltage offset signal responsive to temperature; and at least one differential amplifier operative to receive said at least one voltage offset signal and provide a temperature corrected output voltage signal responsive to said at least one input voltage signal, wherein said at least one differential amplifier is communicatively coupled to both said at least one first gain stage and said at least one second gain stage.